Fukuoka is great, very livable and summer is a good time to go. Things that are fun:
A Hawks game (baseball): great atmosphere
Rent a bike and ride from the Hawks stadium all along the coast. You’ll find cool things like ancient fortress wall ruins, beaches, etc.
There are a lot of beaches in the area.
People are very friendly.
There is this cool building near the center of town that looks like a pyramid. You can walk up it and there is a lot of greenery and places to sit and see the city view. A good place for a stroll or picnic.
There is a temple where, at night, you can put in like 100 yen and it will light up this display. No idea how to explain this better, but it is very unique.
Anyways, you’ll have a good time.